1. Field of the Invention
This invention relates to porous semiconductor material such as silicon in particular, although not exclusively, to methods of producing such material and to devices incorporating it.
2. Discussion of Prior Art
In recent years great interest and considerable research and development activity has been generated in response to the discovery of visible luminescence at room temperature from porous silicon. There have been a substantial number of publications in the scientific literature and also patent applications. See for example the 1992 Fall Meeting of the Materials Research Society, the symposium being entitled "Microcrystalline Semiconductors..Materials Science and Devices", Nov. 30 to Dec. 4 1992. International Patent Applicant No PCT/GB90/01901 published as WO 91/09420 relates to porous silicon having luminescent properties by virtue of its containing silicon quantum wires. Bulk (ie non-porous) silicon has very poor luminescence efficiency because it has indirect gap band structure. Highly porous silicon containing silicon quantum wires has much greater luminescence efficiency, and the luminescence emission is of shorter wavelength. Luminescence is associated with quantum confinement of charge carriers within quantum wires of which the porous silicon is composed.
Experience has shown that the luminescence properties of porous silicon improve both with increasing porosity and with increasing resistivity of the original p-type bulk silicon starting material from which the porous silicon is produced. However, the structural properties of conventionally produced porous silicon degrade with both increasing porosity and increasing starting material resistivity. In Appl. Phys. Lett. Vol 60 (18), pages 2285-2287, May 4, 1992, Friedersdorf et al. discuss the influence of stress on porous silicon luminescence. They show in FIG. 2 an optical micrograph of porous silicon exhibiting "cellular structure", this being the crazing and cracking of porous silicon material. A substantial degree of delamination or peeling of the porous silicon occurs. This makes high porosity porous silicon structurally unsuitable for luminescent device applications. It is too mechanically weak at porosities high enough to provide useful luminescence. Experience shows that porous silicon produced in accordance with International Patent Application No PCT/GB90/01901 begins to exhibit crazing and partial disintegration at porosities above 90% for bulk silicon starting material of resistivity not greater than 10.sup.-2 ohm cm (p.sup.+ type) and porous layer thickness of 4 .mu.m or greater. Here the porosity is determined gravimetrically assuming no shrinkage during production. The situation is worse with higher resistivity p type silicon starting material (p.sup.-, 1 ohm cm). Here crazing and partial disintegration occur at gravimetric porosities above 80% and layer thicknesses similar to the p.sup.+ equivalents.
Difficulties in producing high porosity silicon are also shown by Lehmann et al., Mat. Res. Soc. Symp. Proc. Vol. 283, pages 27-32, 1993. FIG. 6 of this article demonstrates crack evolution and shrinkage during production. Similar effects were also mentioned in a paper by Beale et al. J. Cryst. Growth, Vol. 73, p622 onwards 1985. This paper describes low density porous silicon films which craze and peel during production. In a very recent paper by Grivickas et al., Thin Solid Films, Vol 235, p.234, 1993, it is stated that the thickest porous silicon films broke off from the substrate and disintegrated into small pieces. It is consequently a long felt want to provide high porosity semiconductor material such as silicon with good structural characteristics.
Luminescent porous silicon layers having thicknesses in the range 20 .mu.m to 80 .mu.m are described by Badoz et al. in the Materials Research Society Syposium Procedings, Volume 283, 1993, pages 97 to 108. Similar porous slicon material is reported by Sagnes et al. in Applied Physics Letters, Volume 62(10), 1993, pages 1155-1157. Prior art techniques for the fabrication of thick luminescent porous silicon layers are known to result in structures in which only a top layer of the porous silicon is luminescent.